Three companies have lodged formal claims with the European Commission that Google is acting anti-competitively. Two of the complaints relate to the way that it ranks web pages. The Commission has asked Google to explain its actions.

A high ranking in Google's search engine is vital for many companies, particularly in Europe where the search engine is almost totally dominant. Now three companies have alleged that the way Google ranks their sites is a breach of competition law.

UK search engine Foundem claims that the methods used by Google to determine which sites should be shown at the top of the list for a search term discriminate against it because as a search engine it is a competitor to Google. The issue is vitally important because those lists of search results can run into the millions of items.

Foundem claims that 'penalty filters' used by Google's system to root out unwanted traffic are trained against competing firms.

"Whereas these penalties used to be reserved for spam, or sites caught attempting to cheat Google’s algorithms, they are now increasingly targeted at perfectly legitimate vertical search and directory services," it says in a statement on its website. "It may not be coincidence that, collectively, these services present a nascent competitive threat to Google’s share of online advertising revenues."

Google said that a complaint from a French search engine ejustice.fr "seems to echo these concerns".

"We understand how important rankings can be to websites, especially commercial ones, because a higher ranking typically drives higher volumes of traffic," said Google competition counsel Julia Holtz. "We are also the first to admit that our search is not perfect, but it's a very hard computer science problem to crack. Imagine having to rank the 272 million possible results for a popular query like the iPod on a 14 by 12 screen computer screen in just a few milliseconds. It's a challenge we face millions of times each day."

"Our algorithms aim to rank first what people are most likely to find useful and we have nothing against vertical search sites - indeed many vertical search engines like Moneysupermarket.com, Opodo and Expedia typically rank high in Google's results," said Holtz.

The third company to make a complaint is price comparison site Ciao, which was bought by Microsoft in 2008. Google said that it worked well with Ciao until the Microsoft purchase.

"Ciao … were a long-time AdSense partner of Google's, with whom we always had a good relationship," said Holtz. "However, after Microsoft acquired Ciao! in 2008 (renaming it Ciao! from Bing) we started receiving complaints about our standard terms and conditions. They initially took their case to the German competition authority, but it now has been transferred to Brussels."

The Commission, which investigates and takes action against companies that break EU competition law, confirmed that it had received the complaints.

"The Commission can confirm that it has received three complaints against Google which it is examining," said a Commission statement. "The Commission has not opened a formal investigation for the time being. As is usual when the Commission receives complaints, it informed Google earlier this month and asked the company to comment on the allegations."

Holtz denied that Google's behaviour was designed to stop competitor sites from gaining traffic through its search engine.

"The question [the cases] ultimately pose is whether Google is doing anything to choke off competition or hurt our users and partners," she said. "This is not the case. We always try to listen carefully if someone has a real concern and we work hard to put our users' interests first and to compete fair and square in the market. We believe our business practices reflect those commitments."

Google's search engine is responsible for 91% of internet searches worldwide, according to StatCounter figures for the past 12 months, and its ubiquity has been the cause of similar legal claims before as companies take it to task for not giving their services enough prominence in search results.

A child-related search engine called KinderStart sued Google in the US, claiming that its failure to put it high enough in search results was an interference with the company's constitutional rights to free speech and was anti-competitive.

"[KinderStart's] allegations are vague and ambiguous, and KinderStart makes only general claims as to the type of injury it allegedly suffered … KinderStart still has failed to identify a provably false statement," said the ruling.